Method of compensating for malperforming nozzles in a multitone inkjet printer

Abstract

The present invention generally relates to a method and apparatus for compensating for malperforming or inoperative ink nozzles in a multitone ink jet printhead so that high quality images are printed although some ink nozzles are malperforming or inoperative. Multitone printing is effected by printing a variety of droplets of varying volumes at a given pixel location. In compensating for a malperforming nozzle, a swath data signal is modified and one or more functional nozzles are assigned the printing data for a malperforming nozzle such that the volume of ink ultimately printed at pixel locations is substantially unchanged and the resulting image is free from degradation. Additionally, malperforming nozzles may be assigned values which represent the degree of image degradation that would be caused by printing with the malperforming nozzles, such that these values may be taken into consideration during the process of modifying the swath data signal for complementary recording.

What is claimed is:  
     
       1. A method of compensating for at least one malperforming nozzle in an inkjet printing device having a printhead with a plurality of nozzles which are organized in nozzle groups, each nozzle group including a first nozzle which prints along a first row of image pixels, and at least a second nozzle which is capable of printing along substantially the same row of image pixels as the first nozzle, said nozzle groups adapted to print multiple ink droplets of various sizes at a single pixel location using two or more states on a receiver in response to a swath data signal, wherein each state corresponds to a volume of ink that is desired to be emitted by a nozzle and a zero state corresponds to no ejection of an ink drop, comprising the steps of: 
       a) relating each optical density at an image pixel to a plurality of sets of states including one state corresponding to a first droplet volume and a second state corresponding to a second, larger droplet volume, wherein said plurality of said sets of states result in substantially the same optical density and each of said sets of states are sequenced by the number of zero states in the set;  
       b) assigning a set of states to the image pixel wherein the number of zero states is at least equal to the number of malperforming nozzles in the nozzle group;  
       c) receiving the swath data signal and assigning a zero state in a set of states corresponding to an optical density on the receiver to each malperforming nozzle in the nozzle group, thereby producing a modified swath data signal that assigns the printing data for each nonperforming nozzle to one or more performing nozzles such that the resulting inkjet printing does not result in substantial degradation of the appearance of the image pixels; and,  
       d) printing the image pixels according to the modified swath data signal and producing substantially the same optical density as when each nozzle in the nozzle group is performing.  
     
     
       2. The method of  claim 1  wherein compensating for the malperforming nozzles includes compensating for inoperative nozzles. 
     
     
       3. The method of  claim 1  wherein compensating for the malperforming nozzles includes compensating for nozzles that eject ink drops with ink volumes outside of a specified ink volume range. 
     
     
       4. The method of  claim 1  wherein compensating for the malperforming nozzles includes compensating for intermittently operative nozzles. 
     
     
       5. A method of compensating for at least one malperforming nozzle in an inkjet printing device having a printhead with a plurality of nozzles which are organized in nozzle groups, each nozzle group including a first nozzle which prints along a first row of image pixels, and at least a second nozzle which is capable of printing along substantially the same row of image pixels as the first nozzle, said nozzle groups adapted to print multiple ink droplets of various sizes at a single pixel location using two or more states on a receiver in response to a swath data signal, wherein each state corresponds to a volume of ink that is desired to be emitted by a nozzle and a zero state corresponds to no ejection of an ink drop, comprising the steps of: 
       a) relating each optical density at an image pixel to a plurality of sets of states including one state corresponding to a first droplet volume and a second state corresponding to a second, larger droplet volume, wherein said plurality of said sets of states result in substantially the same optical density and each of said sets of states are sequenced by the number of zero states in the set;  
       b) detecting the malperforming nozzles using a nozzle performance detector;  
       c) assigning a set of states to the image pixel wherein the number of zero states is at least equal to the number of malperforming nozzles in the nozzle group;  
       d) receiving the swath data signal and assigning a zero state in a set of states corresponding to an optical density on the receiver to each malperforming nozzle in the nozzle group, thereby producing a modified swath data signal that assigns the printing data for each nonperforming nozzle to one or more performing nozzles such that the resulting inkjet printing does not result in substantial degradation of the appearance of the image pixels; and,  
       e) printing the image pixels according to the modified swath data signal and producing substantially the same optical density as when each nozzle in the nozzle group is performing.  
     
     
       6. The method of  claim 5  wherein step b) includes a nozzle performance detector that is an optical detector. 
     
     
       7. The method of  claim 5  wherein step b) includes generating nozzle performance data in response to a printed test pattern. 
     
     
       8. A method of compensating for at least one malperforming nozzle in an inkjet printing device having a printhead with a plurality of nozzles which are organized in nozzle groups, each nozzle group including a first nozzle which prints along a first row of image pixels, and at least a second nozzle which is capable of printing along substantially the same row of image pixels as the first nozzle, said nozzles adapted to printing optical densities at the image pixels using two or more states on a receiver in response to a swath data signal, wherein each state corresponds to a volume of ink that is desired to be emitted by a nozzle and a zero state corresponds to no ejection of an ink drop, comprising the steps of: 
       a) relating each optical density at an image pixel to a plurality of sets of states, each of said sets of states being sequenced by the number of zero states in the set;  
       b) assigning a set of states to the image pixel wherein the number of zero states is at least equal to the number of malperforming nozzles in the nozzle group, and wherein each state is assigned a state importance value;  
       c) assigning a nozzle malperforming value to each nozzle, said nozzle malperforming value indicating the relative image quality penalty of using the given nozzle compared to other nozzles;  
       d) computing a modified swath data signal in response to the swath data signal, the state importance value, and the nozzle malperformance value; and,  
       e) printing the image pixels according to the modified swath data signal.